A wide variety of instrumentation systems and surgical techniques have been developed to stabilize and correct spinal conditions and/or deformities, including systems and techniques for correcting degenerative disc disease, spondylolisthesis, spinal deformities, or other spinal conditions through minimally invasive or invasive spinal surgery. In many cases, spinal surgery may include a desire to stabilize a portion of the spine to allow bone or other tissue growth between vertebral bodies, such that a portion of the spine is stabilized or “fused” into a solitary unit and/or specified shape. Commonly known as spinal fusion, this type of stabilization is a commonly-accepted surgical procedure which promotes fusing or growing together of two or more vertebrae in the spine.
The spine is a series of individual bones called vertebrae, separated by cartilaginous disks. The spine includes seven cervical (neck) vertebrae, 12 thoracic (chest) vertebrae, five lumbar (lower back) vertebrae, and the fused vertebrae in the sacrum and coccyx that help to form the hip region. While the shapes of individual vertebrae differ among these regions, each is essentially a short hollow tube containing the bundle of nerves known as the spinal cord. Individual nerves, such as those carrying messages to the arms or legs, enter and exit the spinal cord through gaps between vertebrae. The spinal disks act as shock absorbers, cushioning the spine, and preventing individual bones from contacting each other. Disks also help to hold the vertebrae together. The weight of the upper body is transferred through the spine to the hips and the legs. The spine is held upright through the work of the back muscles, which are attached to the vertebrae. While the normal spine has no significant side-to-side curve, it does have a series of front-to-back curves, giving it a gentle “S” shape. The spine curves in at the lumbar region, back out at the thoracic region, and back in at the cervical region.
One type of spinal fusion procedure is a posterior spinal fusion surgery. This procedure is performed posteriorly, or from the back of the patient, as opposed to anteriorly, or through the abdomen. There are many surgical fusion procedures performed with pedicle screw fixation, which can include (among others) posterolateral gutter fusion surgery, posterior lumbar interbody fusion (“PLIF”) surgery and transforaminal lumbar interbody fusion (“TLIF”) surgery. Moreover, there are many approaches and systems for performing posterior spinal surgery. Various exemplary systems can include titanium construction that are compatible with current CT and MRI scanning technology, low profile implant systems, top-loading and top-tightening systems, and other parameters. Some systems also include cross-connectors that allow an implant to be applied across a dual-rod construct for additional strength and stabilization.
A wide variety of popular systems for spinal stabilization and/or fusion employ the use of pedicle or other type screws and rods, in which screw assemblies can be secured into the bony structures of the patient's vertebrae, and one or more rods or other devices are connected between the screw assemblies, typically disposed longitudinally along the length of the spinal segment to anchor vertebral bodies relative to each other. The rods can assume a wide variety of shapes (i.e., straight, curved or irregularly shaped), various positions (i.e., posterior, anterior and/or lateral) and/or configurations (including the use of cross-arms or cross-connectors, where desired) according to the patient's anatomy and/or the correction desired. In many cases, the patient's anatomy and/or the desired surgical correction may require aligning one or more rods and associated anchoring screws at numerous different angles and/or orientations along the length of the portion of the treated spinal segment.
A unique challenge for bony fixation can arise when the lower levels of the spine are involved, as loosening or breakage of pedicle screws in the lower levels of the spine is not infrequent. Pull out and breakage of pedicle screws typically results in failure of the stabilization system. In such a case, as the stabilization is lost, many of the original complaints from the patient return (i.e., pain and numbness of the low back and leg, inability to walk, foot weakness). In addition, the patient may feel pain at the implantation area due to the loose implant.
Where pedicle screws are implanted into the sacral levels of the spine, loosening or breakage of the pedicle screws is often seen in the first sacral vertebra. There are various reasons why pedicle screws suffer a higher than normal rate of failure at the sacral level. Primarily, the sacral screw bears more load than all the other screws in a typical spinal construct, because the S1 vertebra is the point where the lumbar spine and the sacrum intersect. When a lumbosacral stabilization is performed, the majority of the pull-out forces from the screws in the upper vertebral levels are transmitted to the sacrum, which typically acts as a single-piece unit. Consequently, a properly selected and implanted sacral screw should be very strong. But anatomical constraints can often limit the size and permitted anchoring approaches for sacral screws, rendering the S1 pedicle screw fixation weaker than equivalent lumbar pedicle screws. For example, the anterior-posterior diameter of the S1 body is generally shorter than lumbar vertebrae, requiring a shorter screw than compared to the lumbar pedicle screws. Concurrently, if the anatomy allows a longer S1 screw, it is often required to direct the screw medially to the midline, which can be difficult due to regional anatomical constraints and the presence of the iliac bone. The iliac bone can prevent the screw head from being tilted laterally and cause the screw to be placed straight, and can force the surgeon to create a longer skin incision, make a larger lateral soft tissue retraction and/or even require resection of some of the ilium to properly position the screw.
Because the placement of classical pedicle screws at the sacral level is technically difficult and can often increase surgical trauma experienced by the patient, specialized sacral plates and screws have been developed to utilize additional screw fixation to augment the primary anchoring screw. However, many of these devices require an expansion of the surgical field and/or preparation for implantation of a new screw to be performed (i.e., preparation of a new entry point, fluoroscopy, etc.), and it may become necessary to extend the spinal rod for fastening of the rod to the new screw. Moreover, many of these existing sacral fixation systems are rather large and bulky, and the limited modularity and/or flexibility of the components in many of these systems can render the systems difficult for a surgeon to use effectively.